The living bacterium

Bacterial taxonomy

Bacterial cells are small (about 0.25-25 |im) and of spherical, rod or corkscrew shape, and collectively referred to as cocci, bacilli and spirilla (Fig. 8.2a). These cells may be solitary or arranged in filamentous trichomes with or without branching. Most of the bacilli and all of the spirilla possess a whip-like flagellum (one or more per cell), but these are very thin and are rarely preserved.

Bacteria may feed either on preformed organic matter (heterotrophy) or synthesize organic material from inorganic CO2 (autotrophy). Autotrophic feeding may involve inorganic chemical reactions (chemoauto-trophy), including minerals in rocks (chemolitho-autrophy). Others have evolved organic photosynthesis by means of chlorophyll and related pigments in the presence of sunlight, much like green plants (photoauto-trophy). As a group, the bacteria are relatively unaffected by salinity, and have a temperature tolerance of about 0-125°C. Many dislike a pH outside of the range 6.0-9.0 and will die in bright sunlight. Their habitats range from the deep sea (planktonic and benthic) to terrestrial (including deep subterranean) and aerial.

The taxonomy of living bacteria is largely based on staining tests and aspects of biochemistry particularly rRNA sequence data beyond the scope of palaeontology but has fundamental implications for the early evolution of life (see Chapter 6). With the exception of the more highly differentiated cyanobacteria, a morphological classification would prove misleading because similar morphotypes occur in several different orders of bacteria. Similarly bacteria show metabolic versatility and caution has to be applied in using this in classification.

The order Pseudomonadales contains most of the autotrophic bacteria, including the sulphur bacteria which liberate sulphur and sulphates from H2S. Also included are the stalked bacteria (family Caulobactera-ceae) whose fine stalks become encrusted with ferric hydroxide salts from oxidation of dissolved ferrous iron (e.g. Recent Caulobacter, Fig. 8.2e). These organisms hence assist in the formation of bog iron ores. Carboniferous iron pyrites nodules have also yielded the somewhat similar genus Gallionella (Schopf et al. 1965).

Schematic Diagram Bacteria Budding

Fig. 8.2 Bacteria. (a) Basic shapes of bacterial cells (schematic). (b) Precambrian Eobacterium (length 0.6 |m). (c) Recent sheathed iron bacterium Sphaerotilus. (d) Precambrian Sphaerotilus-like form. (e) Recent 'stalked' iron bacterium Caulobacter. (f) Recent 'budding' bacterium Metallogenium. (g) Precambrian Kakabekia. (h) Precambrian Eoastrion. Single scale bar = 10 |m; double scale bar = 100 |m. ((b) Based on Barghoorn & Schopf 1966; (d) based on Karkhanis 1976: (f) and (h) based on Cloud 1976; (g) based on Barghoorn & Tyler 1965.)

Fig. 8.2 Bacteria. (a) Basic shapes of bacterial cells (schematic). (b) Precambrian Eobacterium (length 0.6 |m). (c) Recent sheathed iron bacterium Sphaerotilus. (d) Precambrian Sphaerotilus-like form. (e) Recent 'stalked' iron bacterium Caulobacter. (f) Recent 'budding' bacterium Metallogenium. (g) Precambrian Kakabekia. (h) Precambrian Eoastrion. Single scale bar = 10 |m; double scale bar = 100 |m. ((b) Based on Barghoorn & Schopf 1966; (d) based on Karkhanis 1976: (f) and (h) based on Cloud 1976; (g) based on Barghoorn & Tyler 1965.)

The order Chlamybacteriales, or sheathed bacteria, is also involved in iron ore formation. These have a trichome organization with a sheath that can become encrusted with ferric or manganese oxides much as in the stalked bacteria (e.g. Recent Sphaerotilus, Fig. 8.2c). Similar bacteria may have participated in the formation of the world's most extensive iron ores in the early and mid Precambrian banded iron formations (Fig. 8.2d; see Karkhanis 1976) as well as in the formation of iron pyrite (Schopf et al. 1965).

The budding bacteria (order Hyphomicrobiales) reproduce by budding; that is, threads grow out either from cells or other threads and themselves produce new cells; these bacteria may also be joined by threads, sometimes in aggregates connected to a common surface by stalks. One such Recent genus, Metallogenium (Fig. 8.2f), grows heterotrophically in low-oxygen environments, depositing crusts of manganese oxide around the filaments. Almost identical fossil bacteria, Eoastrion and Kakabekia, occur in the Gunflint Chert flora in association with banded iron formations (Fig. 8.2g,h; see Cloud 1976).

Examples of possible fossil 'true bacteria' (order Eubacteriales) are reported from the 3.1-Ga-old Fig Tree Chert (Eobacterium, Fig. 8.2b). These are tiny bacillus-like structures discovered by electron microscopy of polished chert surfaces (Barghoorn & Schopf 1966), though they may be contaminants. Bacilli may also be involved in the formation of lime mud (Maurin & Noel in Flugel 1977, pp. 136-142) and have been widely reported from various Phanerozoic rocks (Riding 2000).

The Beggiatoales are an order resembling unpig-mented filamentous cyanobacteria and thrive in H2S-rich habitats. Hence, for example, the discovery of Beggiatoa-like remains in Carboniferous iron pyrites (Schopf et al. 1965). Flexibacteria are an even more cyanobacteria-like group with photosynthetic pigments and they dwell alongside cyanophytes in hot springs to be preserved, eventually, in sinters and stromatolites (Walter 1972). Apart from the biological distinction of their not releasing free oxygen, it would be difficult to differentiate flexibacteria and cyanobacteria except on the tenuous basis of cell diameter - the former rarely exceeding 2 |im in diameter and the latter usually exceeding this.

Was this article helpful?

0 0
Freehand Sketching An Introduction

Freehand Sketching An Introduction

Learn to sketch by working through these quick, simple lessons. This Learn to Sketch course will help you learn to draw what you see and develop your skills.

Get My Free Ebook


Post a comment